This invention is limited to batch-type, industrial heat treat furnaces. Batch-type furnaces are generally constructed in one of two ways. For vacuum applications which are typically high temperature furnace applications, the furnace is usually constructed with a water jacket which keeps the outer furnace casing cool to the touch and allows the use of elastomer seals to maintain a very secure, gas tight door connection. This construction is expensive and the cost dramatically increases with the size of the furnace. The other type of furnace construction and the type of construction to which this invention relates uses a thin, gas-tight furnace casing, the interior of which is lined with insulation (fibrous or brick). Seals for such furnace, because of the heat generated, typically comprise a ceramic type rope seal and a variety of door sealing mechanisms exist within the art. Though such seals are effective, they cannot achieve the seal integrity of the elastomer seals used in the vacuum furnace. Recently, this type of furnace, which will hereafter be termed "standard atmosphere furnace" has been operated at higher and higher furnace temperatures despite the lack of a water jacket casing. Such furnace temperatures can exceed slightly over 2000.degree. F. As used herein and throughout this specification, "high temperature" with respect to industrial furnace operation means temperatures in excess of about 1700.degree.-1750.degree. F. This temperature is believed to be a temperature generally accepted within the art as a high temperature furnace application. When standard atmosphere construction furnaces are operated at high temperature, the insulation on the furnace casing must be dramatically increased so that the casing temperature does not exceed about 150.degree. F. Besides the expense of added insulation, because the furnace insulation is applied to the inside of the furnace, the furnace size increases. Thus a larger and more expansive furnace casing is required.
All batch-type furnaces typically use a fan for directing wind mass about the work. The direction of the wind mass is achieved by baffles. It is generally understood within the art that heating of the work is achieved at the low temperature end by convection heat transfer and at the high temperature end by radiation heat transfer because little heat transfer occurs by convection at elevated temperatures. In the furnace of the present invention and as defined in some of the patents incorporated by reference, high speed jet impingement is utilized to boost the temperatures over which convective heat transfer can effectively occur. However, whether high speed jet impingement is or is not used, at high furnace temperatures baffles for directing the wind mass can distort. This distortion can effect the performance of the furnace in that the work may not be heated uniformly because of baffle distortions. The problem is more severe if the wind mass is pumped through jet orifices for heating the work by impingement. Distortion of the baffling then can affect the angle at which the jets impinge the work creating localized, hot spots etc.
Batch furnaces are produced in a variety of shapes. One type of furnace construction typically used in the art can be defined as a cylindrical furnace with a closed end. The work is simply loaded through the open end of the cylindrical tube (which is insulated) and the door seals the furnace interior which is then heated. To achieve heating and cooling of the work, a f an plenum arrangement is typically constructed at the rear end of the furnace.
One specific type of a batch furnace may be generally described as a closed end cylindrical tube, the interior of which is lined with insulation and a rotatable door or vertically slidable door is provided so that work can be placed into and out of the furnace from the open end of the cylindrical tube. In this type furnace, the fan is provided at the closed rear-end and a plenum plate or, as is used herein, a bulkhead is provided adjacent the fan. The bulkhead has a central under-pressure opening and, spaced radially outwardly therefrom, an exit opening so that the wind mass developed by the fan between the bulkhead and the end of the furnace is pumped through the exit opening and returns to the fan through the under-pressure central opening. When such furnaces are operated at high furnace temperatures, thermal expansion and fan pressure cause the bulkhead to expand and contract with the result that the bulkhead distorts. This distortion can adversely influence the wind distribution pattern about the work as discussed generally above.
Of the various types of industrial furnace constructions, there is a furnace construction conventionally referred to as a muffle furnace. In essence, a muffle furnace can be viewed as a tube within a tube. The work which is to be heat treated is placed within a tube and the tube itself is heated which heat is then radiated to heat the work inside the muffle. To avoid any difficulties in semantics, batch coil annealing apparatus can be viewed as a muffle furnace and the imperforate shell disclosed in my prior patents can also be viewed as a muffle. For batch furnace application, the muffle is viewed as a closed end cylindrical tube while a continuous or semicontinuous furnace the muffle is an open-ended tube within a furnace through which work travels. The applications are entirely different and "muffle" as used herein means a muffle furnace for batch work processing. It can be appreciated that the muffle is a thin wall, rather large, imperforate cylindrical tube which at high furnace temperatures tends to distort. This distortion can effect the heat distribution pattern which the outside of the muffle is subjected to. This can produce localized hot spots relative to the work.